Nerve growth factor mediates mechanical allodynia in a mouse model of type 2 diabetes

Defective autophagy contributes to bupivacaine-induced aggravation of painful diabetic neuropathy in db/db mice

Current evidence suggests that hyperactivated or impaired autophagy can lead to neuronal death. The effect of local anesthetics on painful diabetic neuropathy (PDN) and the role of autophagy in the above pathological process remain unclear, warranting further studies. So, PDN models were established by assessing the paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) in leptin gene-mutation (db/db) mice. Wild type (WT) and PDN mice received intrathecal 0.75% bupivacaine or/with intraperitoneal drug treatment (rapamycin or bafilomycin A1). Subsequently, the PWT and PWL were measured to assess hyperalgesia at 6 h, 24 h, 30 h, and 48 h after intrathecal bupivacaine. Also, sensory nerve conduction velocity (SNCV) and motor nerve conduction velocity (MNCV) were measured before and 48 h after intrathecal bupivacaine treatment. The spinal cord tissue of L4-L6 segments and serum were harvested to evaluate the change of autophagy, oxidative stress, oxidative injury, and apoptosis. We found that bupivacaine induced the activation of autophagy but did not affect the pain threshold, SNCV and MNCV in WT mice at predefined time points. Conversely, bupivacaine lowered autophagosome generation and degradation, slowed SNCV and aggravated spinal dorsal horn neuron oxidative injury and hyperalgesia in PDN mice. The autophagy activator (rapamycin) could decrease spinal dorsal horn neuron oxidative injury, alleviate the alterations in SNCV and hyperalgesia in bupivacaine-treated PDN mice. Meanwhile, the autophagy inhibitor (bafilomycin A1) could exacerbate spinal dorsal horn neuron oxidative injury, the alterations in SNCV and hyperalgesia in bupivacaine-treated PDN mice. Our results showed that bupivacaine could induce defective autophagy, slowed SNCV and aggravate spinal dorsal horn neuron oxidative injury and hyperalgesia in PDN mice. Restoring autophagy may represent a potential therapeutic approach against nerve injury in PDN patients with local anesthesia and analgesia.

NGF and Its Role in Immunoendocrine Communication during Metabolic Syndrome

Nerve growth factor (NGF) was the first neurotrophin described. This neurotrophin contributes to organogenesis by promoting sensory innervation and angiogenesis in the endocrine and immune systems. Neuronal and non-neuronal cells produce and secrete NGF, and several cell types throughout the body express the high-affinity neurotrophin receptor TrkA and the low-affinity receptor p75NTR. NGF is essential for glucose-stimulated insulin secretion and the complete development of pancreatic islets. Plus, this factor is involved in regulating lipolysis and thermogenesis in adipose tissue. Immune cells produce and respond to NGF, modulating their inflammatory phenotype and the secretion of cytokines, contributing to insulin resistance and metabolic homeostasis. This neurotrophin regulates the synthesis of gonadal steroid hormones, which ultimately participate in the metabolic homeostasis of other tissues. Therefore, we propose that this neurotrophin's imbalance in concentrations and signaling during metabolic syndrome contribute to its pathophysiology. In the present work, we describe the multiple roles of NGF in immunoendocrine organs that are important in metabolic homeostasis and related to the pathophysiology of metabolic syndrome.

The neuropeptide substance P/neurokinin-1 receptor system and diabetes: From mechanism to therapy

Diabetes is a significant public health issue known as the world's fastest-growing disease condition. It is characterized by persistent hyperglycemia and subsequent chronic complications leading to organ dysfunction and, ultimately, the failure of target organs. Substance P (SP) is an undecapeptide that belongs to the family of tachykinin (TK) peptides. The SP-mediated activation of the neurokinin 1 receptor (NK1R) regulates many pathophysiological processes in the body. There is also a relation between the SP/NK1R system and diabetic processes. Importantly, deregulated expression of SP has been reported in diabetes and diabetes-associated chronic complications. SP can induce both diabetogenic and antidiabetogenic effects and thus affect the pathology of diabetes destructively or protectively. Here, we review the current knowledge of the functional relevance of the SP/NK1R system in diabetes pathogenesis and its exploitation for diabetes therapy. A comprehensive understanding of the role of the SP/NK1R system in diabetes is expected to shed further light on developing new therapeutic possibilities for diabetes and its associated chronic conditions.

Neuronal Inflammation is Associated with Changes in Epidermal Innervation in High Fat Fed Mice

Peripheral neuropathy (PN), a debilitating complication of diabetes, is associated with obesity and the metabolic syndrome in nondiabetic individuals. Evidence indicates that a high fat diet can induce signs of diabetic peripheral PN in mice but the pathogenesis of high fat diet-induced PN remains unknown. PURPOSE: Determine if neuronal inflammation is associated with the development of mechanical hypersensitivity and nerve fiber changes in high fat fed mice. METHODS: Male C57Bl/6 mice were randomized to a standard (Std, 15% kcal from fat) or high fat diet (HF, 54% kcal from fat) for 2, 4, or 8 weeks (n = 11-12 per group). Lumbar dorsal root ganglia were harvested and inflammatory mediators (IL-1α, IL-1β, IL-2, IL-3, IL-4, IL-5, IL-6, IL-10, IL-12p70, IL-17, MCP-1, IFN-γ, TNF-α, MIP-1α, GMCSF, RANTES) were quantified. Hindpaw mechanical sensitivity was assessed using the von Frey test. Intraepidermal nerve fiber density (IENFD) and TrkA nerve fiber density were quantified via immunohistochemistry. RESULTS: After 8 weeks, HF had greater body mass (33.3 ± 1.0 vs 26.7 ± 0.5 g, p < 0.001), fasting blood glucose (160.3 ± 9.4 vs 138.5 ± 3.4 mg/dl, p < 0.05) and insulin (3.58 ± 0.46 vs 0.82 ± 0.14 ng/ml, p < 0.001) compared to Std. IL-1α, RANTES and IL-5 were higher in HF compared to Std after 2 and 4 weeks, respectively (IL-1α: 4.8 ± 1.3 vs 2.9 ± 0.6 pg/mg, p < 0.05; RANTES: 19.6 ± 2.2 vs 13.3 ± 1.2 pg/mg p < 0.05; IL-5: 5.8 ± 0.7 vs 3.1 ± 0.5 pg/mg, p < 0.05). IENFD and TrkA fiber density were also higher in HF vs Std after 4 weeks (IENFD: 39.4 ± 1.2 vs 32.2 ± 1.3 fibers/mm, p < 0.001; TrkA: 30.4 ± 1.8 vs 22.4 ± 1.3 fibers/mm). There were no significant differences in hindpaw sensitivity for Std vs HF. CONCLUSION: Increased inflammatory mediators preceded and accompanied an increase in cutaneous pain sensing nerve fibers in high fat fed mice but was not accompanied by significant mechanical allodynia. Diets high in fat may increase neuronal inflammation and lead to increased nociceptive nerve fiber density.

Long-Term Intranasal Nerve Growth Factor Treatment Favors Neuron Formation in de novo Brain Tissue

Objective

To date, no safe and effective pharmacological treatment has been clinically validated for improving post-stroke neurogenesis. Growth factors are good candidates but low safety has limited their application in the clinic. An additional restraint is the delivery route. Intranasal delivery presents many advantages.

Materials and Methods

A brain lesion was induced in twenty-four rats. Nerve growth factor (NGF) 5 μg/kg/day or vehicle was given intranasally from day 10 post-lesion for two periods of five weeks, separated by a two-week wash out period with no treatment. Lesion volume and atrophy were identified by magnetic resonance imaging (MRI). Anxiety and sensorimotor recovery were measured by behavior tests. Neurogenesis, angiogenesis and inflammation were evaluated by histology at 12 weeks.

Results

Remarkable neurogenesis occurred and was visible at the second and third months after the insult. Tissue reconstruction was clearly detected by T2 weighted MRI at 8 and 12 weeks post-lesion and confirmed by histology. In the new tissue (8.1% of the lesion in the NGF group vs. 2.4%, in the control group at 12 weeks), NGF significantly increased the percentage of mature neurons (19% vs. 7%). Angiogenesis and inflammation were not different in the two groups. Sensorimotor recovery was neither improved nor hampered by NGF during the first period of treatment, but NGF treatment limited motor recovery in the second period.

Interpretation

The first five-week period of treatment was very well tolerated. This study is the first presenting the effects of a long treatment with NGF and has shown an important tissue regeneration rate at 8 and 12 weeks post-injury. NGF may have increased neuronal differentiation and survival and favored neurogenesis and neuron survival through subventricular zone (SVZ) neurogenesis or reprogramming of reactive astrocytes. For the first time, we evidenced a MRI biomarker of neurogenesis and tissue reconstruction with T2 and diffusion weighted imaging.

Nerve and Vascular Biomarkers in Skin Biopsies Differentiate Painful From Painless Peripheral Neuropathy in Type 2 Diabetes

Painful diabetic peripheral neuropathy can be intractable with a major impact, yet the underlying pain mechanisms remain uncertain. A range of neuronal and vascular biomarkers was investigated in painful diabetic peripheral neuropathy (painful-DPN) and painless-DPN and used to differentiate painful-DPN from painless-DPN. Skin biopsies were collected from 61 patients with type 2 diabetes (T2D), and 19 healthy volunteers (HV). All subjects underwent detailed clinical and neurophysiological assessments. Based on the neuropathy composite score of the lower limbs [NIS(LL)] plus seven tests, the T2D subjects were subsequently divided into three groups: painful-DPN (n = 23), painless-DPN (n = 19), and No-DPN (n = 19). All subjects underwent punch skin biopsy, and immunohistochemistry used to quantify total intraepidermal nerve fibers (IENF) with protein gene product 9.5 (PGP9.5), regenerating nerve fibers with growth-associated protein 43 (GAP43), peptidergic nerve fibers with calcitonin gene-related peptide (CGRP), and blood vessels with von Willebrand Factor (vWF). The results showed that IENF density was severely decreased (p < 0.001) in both DPN groups, with no differences for PGP9.5, GAP43, CGRP, or GAP43/PGP9.5 ratios. There was a significant increase in blood vessel (vWF) density in painless-DPN and No-DPN groups compared to the HV group, but this was markedly greater in the painful-DPN group, and significantly higher than in the painless-DPN group (p < 0.0001). The ratio of sub-epidermal nerve fiber (SENF) density of CGRP:vWF showed a significant decrease in painful-DPN vs. painless-DPN (p = 0.014). In patients with T2D with advanced DPN, increased dermal vasculature and its ratio to nociceptors may differentiate painful-DPN from painless-DPN. We hypothesized that hypoxia-induced increase of blood vessels, which secrete algogenic substances including nerve growth factor (NGF), may expose their associated nociceptor fibers to a relative excess of algogens, thus leading to painful-DPN.

Poly (ADP-ribose) polymerase 1-mediated defective mitophagy contributes to painful diabetic neuropathy in the db/db model

Studies have shown that poly (ADP-ribose) polymerase 1 (PARP1) was involved in the pathological process of diabetes. Mitophagy is widely acknowledged to be a key regulatory process in maintaining reactive oxygen species homeostasis via lysosome degradation of damaged mitochondria. However, the regulatory role of PARP1 in mitophagy-related mitochondrial oxidative injury and progression of painful diabetic neuropathy (PDN) is unclear. In this study, we studied the in vitro and in vivo mechanisms of PARP1-mediated mitophagy blockade in a leptin gene-mutation (db/db) mouse model of PDN. Db/db mice models of PDN were established by assessing the sciatic nerve conduction velocity (SNCV), mechanical withdrawal threshold (MWT), and thermal withdrawal latency (TWL). The results showed that PARP1 activity and mitochondrial injury of dorsal root ganglion (DRG) neurons were increased, and mitophagy was impaired in PDN mice. PARP1 was found to mediate the impairment of mitophagy in DRG neurons isolated from PDN mice. PARP1 inhibitors (PJ34 or AG14361) attenuated diabetes-induced peripheral nerve hyperalgesia, restored DRG neuron mitophagy function and decreased mitochondrial oxidative injury. Mitophagy impairment induced by lysosome deacidificant (DC661) aggravated diabetes-induced DRG neuron mitochondrial oxidative stress and injury. Taken together, our data revealed that PARP1 induced defective mitophagy of DRG neurons is a key mechanism in diabetes-induced peripheral neuropathic injury. Inhibition of PARP1 and restoration of mitophagy function are potential therapeutic targets for PDN.

Spinal cord-wide structural disruption in type 2 diabetes rescued by exenatide "a glucagon-like peptide-1 analogue" via down-regulating inflammatory, oxidative stress and apoptotic signaling pathways

The mechanisms of spinal cord-wide structural and functional disruption in diabetic patients remain elusive. This study evaluated histopathological alterations of the spinal cord cytoarchitecture in T2DM model of rats and assessed the potential ameliorating effect of exenatide "a potent GLP-1 analogue". Thirty male rats were allocated into three groups; I (control), II (Diabetic): T2DM was induced by high fat diet for 8 weeks followed by a single I.P injection of STZ (25 mg/kg BW) and III (Diabetic/Exenatide): T2DM rats injected with exenatide (10 μg/Kg, S.C. twice daily for 2 weeks). Neurobehavioral sensory and motor tests were carried out and glycemic control biomarkers and indices of insulin resistance and sensitivity were measured. In addition, the spinal cord was processed for histological and immunohistochemical studies besides assessing its tissue homogenate levels of pro-inflammatory/anti-inflamatory cytokines and oxidant/antioxidant biomarkers. Moreover, RT-qPCR was performed to measure the expression of proapoptotic/antiapoptotic and neurotrophic genes. The diabetic rats exhibited thermal hyperalgesia, mechanical allodynia and decreased locomotor activity along with increased serum glucose, insulin, HbA1c, HOMA-IR while, quantitative insulin sensitivity check index (QUICKI) was decreased. Also, IL-1β NF-kB, MDA increased while IL-10, SOD activity and β-endorphin decreased in the spinal tissue. Up regulation of caspase-3 and down regulation of Bcl-2, nerve growth factor (NGF) and glial cell-derived neurotrophic (GDNF) in diabetic rats. Also, they exhibited histopathological changes and increased CD68 positive microglia and Bax immunoreactivity in the spinal cord. Subsequent to exenatide treatment, most biomolecular, structural and functional impairments of the spinal cord were restored in the diabetic rats. In conclusion, the neuro-modulating effect of exenatide against diabetic-induced spinal cord affection warrants the concern about its therapeutic relevance in confronting the devastating diabetic neuropathic complications.

An Insight into Potential Pharmacotherapeutic Agents for Painful Diabetic Neuropathy

Diabetes is the 4th most common disease affecting the world's population. It is accompanied by many complications that deteriorate the quality of life. Painful diabetic neuropathy (PDN) is one of the debilitating consequences of diabetes that effects one-third of diabetic patients. Unfortunately, there is no internationally recommended drug that directly hinders the pathological mechanisms that result in painful diabetic neuropathy. Clinical studies have shown that anticonvulsant and antidepressant therapies have proven fruitful in management of pain associated with PDN. Currently, the FDA approved medications for painful diabetic neuropathies include duloxetine, pregabalin, tapentadol extended release, and capsaicin (for foot PDN only). The FDA has also approved the use of spinal cord stimulation system for the treatment of diabetic neuropathy pain. The drugs recommended by other regulatory bodies include gabapentin, amitriptyline, dextromethorphan, tramadol, venlafaxine, sodium valproate, and 5 % lidocaine patch. These drugs are only partially effective and have adverse effects associated with their use. Treating painful symptoms in diabetic patient can be frustrating not only for the patients but also for health care workers, so additional clinical trials for novel and conventional treatments are required to devise more effective treatment for PDN with minimal side effects. This review gives an insight on the pathways involved in the pathogenesis of PDN and the potential pharmacotherapeutic agents. This will be followed by an overview on the FDA-approved drugs for PDN and commercially available topical analgesic and their effects on painful diabetic neuropathies.

Inhibitory effects of xanthine oxidase inhibitor, topiroxostat, on development of neuropathy in db/db mice

Inflammation and oxidative stress contribute to the pathophysiology of diabetic neuropathy. According to recent evidence, the modulation of macrophage polarization in peripheral nerves represents a potential therapeutic target for diabetic neuropathy. Xanthine oxidase, which is a form of xanthin oxidoreductase, is the rate-limiting enzyme that catalyzes the degradation of hypoxanthine and xanthine into uric acid. Activation of xanthine oxidase promotes oxidative stress and macrophage activation. A preclinical study reported the beneficial effects of xanthine oxidase inhibitors on peripheral nerve dysfunction in experimental models of diabetes. However, the detailed mechanisms remain unknown. In this study, we examined the effect of the xanthine oxidase inhibitor topiroxostat on macrophage polarization and peripheral neuropathy in an obese diabetic model, db/db mice. First, the effects of xanthine oxidase inhibitors on cultured macrophages and dorsal root ganglion neurons exposed to xanthine oxidase were assessed. Furthermore, five-week-old db/db mice were administered the xanthine oxidase inhibitors topiroxostat [1 mg/kg/day (dbT1) or 2 mg/kg/day (dbT2)] or febuxostat [1 mg/kg (dbF)]. Glucose metabolism and body weight were evaluated during the experimental period. At 4 and 8 weeks of treatment, peripheral nerve functions such as nerve conduction velocities, thermal thresholds and pathology of skin and sciatic nerves were evaluated. The mRNA expression of molecules related to inflammation and oxidative stress was also measured in sciatic nerves. Untreated db/db mice and the nondiabetic db strain (db/m) were studied for comparison. An in vitro study showed that topiroxostat suppressed macrophage activation and proinflammatory but not anti-inflammatory polarization, and prevented the reduction in neurite outgrowth from neurons exposed to xanthine oxidase. Neuropathic changes exemplified by delayed nerve conduction and reduced intraepidermal nerve fiber density developed in db/db mice. These deficits were significantly prevented in the treated group, most potently in dbT2. Protective effects were associated with the suppression of macrophage infiltration, cytokine expression, and oxidative stress in the sciatic nerve and decreased plasma xanthine oxidoreductase activity. Our results revealed the beneficial effects of the xanthine oxidase inhibitor topiroxostat on neuropathy development in a mouse model of type 2 diabetes. The suppression of proinflammatory macrophage activation and oxidative stress-induced damage were suggested to be involved in this process.

Impact of Hepatoma-Derived Growth Factor Blockade on Resiniferatoxin-Induced Neuropathy

Resiniferatoxin is an ultrapotent capsaicin analog that mediates nociceptive processing; treatment with resiniferatoxin can cause an inflammatory response and, ultimately, neuropathic pain. Hepatoma-derived growth factor, a growth factor related to normal development, is associated with neurotransmitters surrounding neurons and glial cells. Therefore, the study aims to investigate how blocking hepatoma-derived growth factor affects the inflammatory response in neuropathic pain. Serum hepatoma-derived growth factor protein expression was measured via ELISA. Resiniferatoxin was administrated intraperitoneally to induce neuropathic pain in 36 male Sprague-Dawley rats which were divided into three groups (resiniferatoxin+recombinant hepatoma-derived growth factor antibody group, resiniferatoxin group, and control group) (n = 12/group). The mechanical threshold response was tested with calibration forceps. Cell apoptosis was measured by TUNEL assay. Immunofluorescence staining was performed to detect apoptosis of neuron cells and proliferation of astrocytes in the spinal cord dorsal horn. RT-PCR technique and western blot were used to measure detect inflammatory factors and protein expressions. Serum hepatoma-derived growth factor protein expression was higher in the patients with sciatica compared to controls. In resiniferatoxin-group rats, protein expression of hepatoma-derived growth factor was higher than controls. Blocking hepatoma-derived growth factor improved the mechanical threshold response in rats. In dorsal root ganglion, blocking hepatoma-derived growth factor inhibited inflammatory cytokines. In the spinal cord dorsal horn, blocking hepatoma-derived growth factor inhibited proliferation of astrocyte, apoptosis of neuron cells, and attenuated expressions of pain-associated proteins. The experiment showed that blocking hepatoma-derived growth factor can prevent neuropathic pain and may be a useful alternative to conventional analgesics.

Interleukin-10 Reduces Neurogenic Inflammation and Pain Behavior in a Mouse Model of Type 2 Diabetes

Purpose

Neurogenic inflammation is a major component of chronic neuropathic pain. Previously, we established the db/db mouse as an animal model of painful diabetic neuropathy (PDN) of type 2 diabetes. In the current study, we investigate the roles of interleukin (IL)-10, an anti-inflammatory cytokine, in the development of neurogenic inflammation and pain behavior in db/db mouse.

Materials and Methods

We first studied IL-10 expression in lumbar dorsal root ganglion (LDRG) neurons of db/db mice using immunohistochemistry, immunoblots, and reverse transcription polymerase chain reaction during the period of pain behavior (from 8 to 16 wk of age). To determine if the reduced IL-10 expression mediates the mechanical allodynia in db/db mice, we administered recombinant mouse IL-10 or saline (control) intraperitoneally to control db/+ and db/db mice starting at 8 wk of age. IL-10 treatment was repeated every other day for 2 wk until the mice reached 10 wk of age.

Results

During the period of pain behavior, reduction of IL-10 protein and gene expression was detected in LDRG of db/db mice. Treatment with recombinant IL-10, from 8 to 10 wk of age, alleviates pain behaviors in db/db mice without affecting other diabetic parameters. In parallel, IL-10 treatment reduced the upregulation of nerve growth factor (NGF), inducible nitric oxide synthase (iNOS), tumor necrosis factor (TNF)-α, and high-affinity NGF receptor (Trk A) in LDRG, as well as the numbers of iNOS-positive Langerhans cells and CD-68-positive dermal dendritic cells in the hind-foot-pad skin in db/db mice.

Conclusion

Our findings suggest that the reduction in neuronal IL-10 increases inflammatory phenomena, ultimately contributing to PDN. These results suggest that the dysregulation of cytokine-mediated inflammation contributes to the development of PDN in db/db mice. Targeting this pathophysiologic mechanism could be an effective approach for treating PDN in type 2 diabetes.

SP6616 as a Kv2.1 inhibitor efficiently ameliorates peripheral neuropathy in diabetic mice

BACKGROUND

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes severely afflicting the patients, while there is yet no effective medication against this disease. As Kv2.1 channel functions potently in regulating neurological disorders, the present work was to investigate the regulation of Kv2.1 channel against DPN-like pathology of DPN model mice by using selective Kv2.1 inhibitor SP6616 (ethyl 5-(3-ethoxy-4-methoxyphenyl)-2-(4-hydroxy-3-methoxybenzylidene)-7-methyl-3-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate) as a probe.

METHODS

STZ-induced type 1 diabetic mice with DPN (STZ mice) were defined at 12 weeks of age (4 weeks after STZ injection) through behavioral tests, and db/db (BKS Cg-m+/+Leprdb/J) type 2 diabetic mice with DPN (db/db mice) were at 18 weeks of age. SP6616 was administered daily via intraperitoneal injection for 4 weeks. The mechanisms underlying the amelioration of SP6616 on DPN-like pathology were investigated by RT-PCR, western blot and immunohistochemistry technical approaches against diabetic mice, and verified against the STZ mice with Kv2.1 knockdown in dorsal root ganglion (DRG) tissue by injection of adeno associated virus AAV9-Kv2.1-RNAi. Amelioration of SP6616 on the pathological behaviors of diabetic mice was assessed against tactile allodynia, thermal sensitivity and motor nerve conduction velocity (MNCV).

FINDINGS

SP6616 treatment effectively ameliorated the threshold of mechanical stimuli, thermal sensitivity and MNCV of diabetic mice. Mechanism research results indicated that SP6616 suppressed Kv2.1 expression, increased the number of intraepidermal nerve fibers (IENFs), improved peripheral nerve structure and vascular function in DRG tissue. In addition, SP6616 improved mitochondrial dysfunction through Kv2.1/CaMKKβ/AMPK/PGC-1α pathway, repressed inflammatory response by inhibiting Kv2.1/NF-κB signaling and alleviated apoptosis of DRG neuron through Kv2.1-mediated regulation of Bcl-2 family proteins and Caspase-3 in diabetic mice.

INTERPRETATION

Our work has highly supported the beneficial of Kv2.1 inhibition in ameliorating DPN-like pathology and highlighted the potential of SP6616 in the treatment of DPN.

FUNDING

Please see funding sources.

Deficiency of glucagon gene-derived peptides induces peripheral polyneuropathy in mice

Although diabetic polyneuropathy (DPN) is the commonest diabetic complication, its pathology remains to be clarified. As previous papers have suggested the neuroprotective effects of glucagon-like peptide-1 in DPN, the current study investigated the physiological indispensability of glucagon gene-derived peptides (GCGDPs) including glucagon-like peptide-1 in the peripheral nervous system (PNS). Neurological functions and neuropathological changes of GCGDP deficient (gcg-/-) mice were examined. The gcg-/- mice showed tactile allodynia and thermal hyperalgesia at 12-18 weeks old, followed by tactile and thermal hypoalgesia at 36 weeks old. Nerve conduction studies revealed a decrease in sensory nerve conduction velocity at 36 weeks old. Pathological findings showed a decrease in intraepidermal nerve fiber densities. Electron microscopy revealed a decrease in circularity and an increase in g-ratio of myelinated fibers and a decrease of unmyelinated fibers in the sural nerves of the gcg-/- mice. Effects of glucagon on neurite outgrowth were examined using an ex vivo culture of dorsal root ganglia. A supraphysiological concentration of glucagon promoted neurite outgrowth. In conclusion, the mice with deficiency of GCGDPs developed peripheral neuropathy with age. Furthermore, glucagon might have neuroprotective effects on the PNS of mice. GCGDPs might be involved in the pathology of DPN.

Exaggerated exercise pressor reflex in type 2 diabetes: Potential role of oxidative stress

Type 2 diabetes mellitus (T2DM) leads to exaggerated cardiovascular responses to exercise, in part due to an exaggerated exercise pressor reflex. Accumulating data suggest excessive oxidative stress contributes to an exaggerated exercise pressor reflex in cardiovascular-related diseases. Excessive oxidative stress is also a primary underlying mechanism for the development and progression of T2DM. However, whether oxidative stress plays a role in mediating the exaggerated exercise pressor reflex in T2DM is not known. Therefore, this review explores the potential role of oxidative stress leading to increased activation of the afferent arm of the exercise pressor reflex. Several lines of evidence support direct and indirect effects of oxidative stress on the exercise pressor reflex. For example, intramuscular ROS may directly and indirectly (by attenuating contracting muscle blood flow) increase group III and IV afferent activity. Oxidative stress is a primary underlying mechanism for the development of neuropathic pain, which in turn is associated with increased group III and IV afferent activity. These are the same type of afferents that evoke muscle pain and the exercise pressor reflex. Furthermore, oxidative stress-induced release of inflammatory mediators may modulate afferent activity. Collectively, these alterations may result in a positive feedback loop that further amplifies the exercise pressor reflex. An exaggerated reflex increases the risk of adverse cardiovascular events. Thus, identifying the contribution of oxidative stress could provide a potential therapeutic target to reduce this risk in T2DM.

Neuropathy and neural plasticity in the subcutaneous white adipose depot

The difficulty in obtaining as well as maintaining weight loss, together with the impairment of metabolic control in conditions like diabetes and cardiovascular disease, may represent pathological situations of inadequate neural communication between the brain and peripheral organs and tissues. Innervation of adipose tissues by peripheral nerves provides a means of communication between the master metabolic regulator in the brain (chiefly the hypothalamus), and energy-expending and energy-storing cells in the body (primarily adipocytes). Although chemical and surgical denervation studies have clearly demonstrated how crucial adipose tissue neural innervation is for maintaining proper metabolic health, we have uncovered that adipose tissue becomes neuropathic (ie: reduction in neurites) in various conditions of metabolic dysregulation. Here, utilizing both human and mouse adipose tissues, we present evidence of adipose tissue neuropathy, or loss of proper innervation, under pathophysiological conditions such as obesity, diabetes, and aging, all of which are concomitant with insult to the adipose organ as well as metabolic dysfunction. Neuropathy is indicated by loss of nerve fiber protein expression, reduction in synaptic markers, and lower neurotrophic factor expression in adipose tissue. Aging-related adipose neuropathy particularly results in loss of innervation around the tissue vasculature, which cannot be reversed by exercise. Together with indications of neuropathy in muscle and bone, these findings underscore that peripheral neuropathy is not restricted to classic tissues like the skin of distal extremities, and that loss of innervation to adipose may trigger or exacerbate metabolic diseases. In addition, we have demonstrated stimulation of adipose tissue neural plasticity with cold exposure, which may ameliorate adipose neuropathy and be a potential therapeutic option to re-innervate adipose and restore metabolic health.

Exaggerated cardiovascular responses to muscle contraction and tendon stretch in UCD type-2 diabetes mellitus rats

Patients with type-2 diabetes mellitus (T2DM) have exaggerated sympathetic activity and blood pressure responses to exercise. However, the underlying mechanisms for these responses, as well as how these responses change throughout disease progression, are not completely understood. For this study, we examined the effect of the progression of T2DM on the exercise pressor reflex, a critical neurocardiovascular mechanism that functions to increase sympathetic activity and blood pressure during exercise. We also aimed to examine the effect of T2DM on reflexive cardiovascular responses to static contraction, as well as those responses to tendon stretch when an exaggerated exercise pressor reflex was present. We evoked the exercise pressor reflex and mechanoreflex by statically contracting the hindlimb muscles and stretching the Achilles tendon, respectively, for 30 s. We then compared pressor and cardioaccelerator responses in unanesthetized, decerebrated University of California Davis (UCD)-T2DM rats at 21 and 31 wk following the onset of T2DM to responses in healthy nondiabetic rats. We found that the pressor response to static contraction was greater in the 31-wk T2DM [change in mean arterial pressure (∆MAP) = 39 ± 5 mmHg] but not in the 21-wk T2DM (∆MAP = 24 ± 5 mmHg) rats compared with nondiabetic rats (∆MAP = 18 ± 2 mmHg; P < 0.05). Similarly, the pressor and the cardioaccelerator responses to tendon stretch were significantly greater in the 31-wk T2DM rats [∆MAP = 69 ± 6 mmHg; change in heart rate (∆HR) = 28 ± 4 beats/min] compared with nondiabetic rats (∆MAP = 14 ± 2 mmHg; ∆HR = 5 ± 3 beats/min; P < 0.05). These findings suggest that the exercise pressor reflex changes as T2DM progresses and that a sensitized mechanoreflex may play a role in exaggerating these cardiovascular responses.NEW & NOTEWORTHY This is the first study to provide evidence that as type-2 diabetes mellitus (T2DM) progresses, the exercise pressor reflex becomes exaggerated, an effect that may be due to a sensitized mechanoreflex. Moreover, these findings provide compelling evidence suggesting that impairments in the reflexive control of circulation contribute to exaggerated blood pressure responses to exercise in T2DM.

Mechanisms underlying mechanical sensitization induced by complement C5a: the roles of macrophages, TRPV1, and calcitonin gene-related peptide receptors

The complement system significantly contributes to the development of inflammatory and neuropathic pain, but the underlying mechanisms are poorly understood. Recently, we identified the signaling pathway responsible for thermal hypersensitivity induced by the complement system component C5a. Here, we examine the mechanisms of another important action of C5a, induction of mechanical hypersensitivity. We found that intraplantar injection of C5a produced a dose-dependent mechanical sensitization and that this effect was blocked by chemogenetic ablation of macrophages in both male and female mice. Knockout of TRPV1 or pretreatment with the TRPV1 antagonists, AMG9810 or 5'-iodoresiniferatoxin (5'-IRTX), significantly reduced C5a-induced mechanical sensitization. Notably, local administration of 5'-IRTX 90 minutes after C5a injection resulted in a slow, but complete, reversal of mechanical sensitization, indicating that TRPV1 activity was required for maintaining C5a-induced mechanical hypersensitivity. This slow reversal suggests that neurogenic inflammation and neuropeptide release may be involved. Indeed, pretreatment with a calcitonin gene-related peptide (CGRP) receptor antagonist (but not an antagonist of the neurokinin 1 receptor) prevented C5a-induced mechanical sensitization. Furthermore, intraplantar injection of CGRP produced significant mechanical sensitization in both wild-type and TRPV1 knockout mice. Taken together, these findings suggest that C5a produces mechanical sensitization by initiating macrophage-to-sensory-neuron signaling cascade that involves activation of TRPV1 and CGRP receptor as critical steps in this process.

Mitochondrial uncoupling has no effect on microvascular complications in type 2 diabetes

Diabetic peripheral neuropathy (DPN), diabetic kidney disease (DKD), and diabetic retinopathy (DR) contribute to significant morbidity and mortality in diabetes patients. The incidence of these complications is increasing with the diabetes epidemic, and current therapies minimally impact their pathogenesis in type 2 diabetes (T2D). Improved mechanistic understanding of each of the diabetic complications is needed in order to develop disease-modifying treatments for patients. We recently identified fundamental differences in mitochondrial responses of peripheral nerve, kidney, and retinal tissues to T2D in BKS-db/db mice. However, whether these mitochondrial adaptations are the cause or consequence of tissue dysfunction remains unclear. In the current study BKS-db/db mice were treated with the mitochondrial uncoupler, niclosamide ethanolamine (NEN), to determine the effects of mitochondrial uncoupling therapy on T2D, and the pathogenesis of DPN, DKD and DR. Here we report that NEN treatment from 6-24 wk of age had little effect on the development of T2D and diabetic complications. Our data suggest that globally targeting mitochondria with an uncoupling agent is unlikely to provide therapeutic benefit for DPN, DKD, or DR in T2D. These data also highlight the need for further insights into the role of tissue-specific metabolic reprogramming in the pathogenesis of diabetic complications.

Diabetic neuropathy research: from mouse models to targets for treatment

Diabetic neuropathy is one of the most serious complications of diabetes, and its increase shows no sign of stopping. Furthermore, current clinical treatments do not yet approach the best effectiveness. Thus, the development of better strategies for treating diabetic neuropathy is an urgent matter. In this review, we first discuss the advantages and disadvantages of some major mouse models of diabetic neuropathy and then address the targets for mechanism-based treatment that have been studied. We also introduce our studies on each part. Using stem cells as a source of neurotrophic factors to target extrinsic factors of diabetic neuropathy, we found that they present a promising treatment.

Characterization of diabetic neuropathy progression in a mouse model of type 2 diabetes mellitus

Diabetes mellitus (DM) is one of most common chronic diseases with an increasing incidence in most countries. Diabetic neuropathy (DN) is one of the earliest and main complications of diabetic patients, which is characterized by progressive, distal-to-proximal degeneration of peripheral nerves. The cellular and molecular mechanisms that trigger DN are highly complex, heterogeneous and not completely known. Animal models have constituted a valuable tool for understanding diabetes pathophysiology; however, the temporal course of DN progression in animal models of type 2 diabetes (T2DM) is not completely understood. In this work, we characterized the onset and progression of DN in BKS diabetic (db/db) mice, including the main functional and histological features observed in the human disease. We demonstrated that diabetic animals display progressive sensory loss and electrophysiological impairments in the early-to-mid phases of the disease. Furthermore, we detected an early decrease in intraepidermal nerve fiber (IENF) density in 18-week-old diabetic mice, which is highly associated with sensory loss and constitutes a reliable marker of DN. Other common histological parameters of DN – like Schwann cells apoptosis and infiltration of CD3⁺ cells in the sciatic nerve – were altered in mid-to-late phases of the disease. Our results support the general consensus that DN evolves from initial functional to late structural changes. This work aimed to characterize the progression of DN in a reliable animal model sharing the main human disease features, which is necessary to assess new therapies for this complex disease. Finally, we also aimed to identify an effective temporal window where these potential treatments could be successfully applied.

Summary: We characterized the main functional and structural diabetic neuropathy features during early-to-late phases of type 2 diabetes mellitus. This study aimed to identify a therapeutic window for new treatments.

Topical thermal therapy with hot packs suppresses physical inactivity-induced mechanical hyperalgesia and up-regulation of NGF

We focused on the analgesic effect of hot packs for mechanical hyperalgesia in physically inactive rats. Male Wistar rats were randomly divided into four groups: control, physical inactivity (PI), PI + sham treatment (PI + sham), and PI + hot pack treatment (PI + hot pack) groups. Physical inactivity rats wore casts on both hind limbs in full plantar flexed position for 4 weeks. Hot pack treatment was performed for 20 min a day, 5 days a week. Although mechanical hyperalgesia and the up-regulation of NGF in the plantar skin and gastrocnemius muscle were observed in the PI and the PI + sham groups, these changes were significantly suppressed in the PI + hot pack group. The present results clearly demonstrated that hot pack treatment was effective in reducing physical inactivity-induced mechanical hyperalgesia and up-regulation of NGF in plantar skin and gastrocnemius muscle.

Transcriptional networks of progressive diabetic peripheral neuropathy in the db/db mouse model of type 2 diabetes: An inflammatory story

Diabetic peripheral neuropathy is the most common complication of diabetes and a source of considerable morbidity. Numerous molecular pathways are linked to neuropathic progression, but it is unclear whether these pathways are altered throughout the course of disease. Moreover, the methods by which these molecular pathways are analyzed can produce significantly different results; as such it is often unclear whether previously published pathways are viable targets for novel therapeutic approaches. In the current study we examine changes in gene expression patterns in the sciatic nerve (SCN) and dorsal root ganglia (DRG) of db/db diabetic mice at 8, 16, and 24 weeks of age using microarray analysis. Following the collection and verification of gene expression data, we utilized both self-organizing map (SOM) analysis and differentially expressed gene (DEG) analysis to detect pathways that were altered at all time points. Though there was some variability between SOM and DEG analyses, we consistently detected altered immune pathways in both the SCN and DRG over the course of disease. To support these results, we further used multiplex analysis to assess protein changes in the SCN of diabetic mice; we found that multiple immune molecules were upregulated at both early and later stages of disease. In particular, we found that matrix metalloproteinase-12 was highly upregulated in microarray and multiplex data sets suggesting it may play a role in disease progression.

Cytokine-mediated inflammation mediates painful neuropathy from metabolic syndrome

Painful neuropathy (PN) is a prevalent condition in patients with metabolic syndrome (MetS). However, the pathogenic mechanisms of metabolic syndrome-associated painful neuropathy (MetSPN) remain unclear. In the current study, high-fat-fed mice (HF mice) were used to study MetSPN. HF mice developed MetS phenotypes, including increased body weight, elevated plasma cholesterol levels, and insulin resistance in comparison with control-fat-fed (CF) mice. Subsequently, HF mice developed mechanical allodynia and thermal hyperalgesia in hind paws after 8 wk of diet treatment. These pain behaviors coincided with increased densities of nociceptive epidermal nerve fibers and inflammatory cells such as Langerhans cells and macrophages in hind paw skin. To study the effect of MetS on profiles of cytokine expression in HF mice, we used a multiplex cytokine assay to study the protein expression of 12 pro-inflammatory and anti-inflammatory cytokines in dorsal root ganglion and serum samples. This method detected the elevated levels of proinflammatory cytokines, including tumor necrosis factor (TNF)-α, and interleukin (IL)-6, IL-1β as well as reduced anti-inflammatory IL-10 in lumbar dorsal root ganglia (LDRG) of HF mice. Intraperitoneal administration of IL-10 reduced the upregulation of pro-inflammatory cytokines and alleviated pain behaviors in HF mice without affecting MetS phenotypes. Our findings suggested targeting HF-induced cytokine dysregulation could be an effective strategy for treating MetSPN.

Shared and distinct lipid-lipid interactions in plasma and affected tissues in a diabetic mouse model

Lipids are ubiquitous metabolites with diverse functions; abnormalities in lipid metabolism appear to be related to complications from multiple diseases, including type 2 diabetes. Through technological advances, the entire lipidome has been characterized and researchers now need computational approaches to better understand lipid network perturbations in different diseases. Using a mouse model of type 2 diabetes with microvascular complications, we examined lipid levels in plasma and in renal, neural, and retinal tissues to identify shared and distinct lipid abnormalities. We used correlation analysis to construct interaction networks in each tissue, to associate changes in lipids with changes in enzymes of lipid metabolism, and to identify overlap of coregulated lipid subclasses between plasma and each tissue to define subclasses of plasma lipids to use as surrogates of tissue lipid metabolism. Lipid metabolism alterations were mostly tissue specific in the kidney, nerve, and retina; no lipid changes correlated between the plasma and all three tissue types. However, alterations in diacylglycerol and in lipids containing arachidonic acid, an inflammatory mediator, were shared among the tissue types, and the highly saturated cholesterol esters were similarly coregulated between plasma and each tissue type in the diabetic mouse. Our results identified several patterns of altered lipid metabolism that may help to identify pathogenic alterations in different tissues and could be used as biomarkers in future research into diabetic microvascular tissue damage.

Nerve growth factor is elevated in the CSF of patients with multiple sclerosis and central neuropathic pain

Central neuropathic pain (CNP) is common and disabling among patients with multiple sclerosis (MS). The pathological mechanisms underlying CNP in MS are not well understood. We explored whether NGF is implicated in the pathogenesis of CNP in MS. We measured NGF concentration in the CSF of 73 patients affected by MS, 15 with and 58 without CNP and 14 controls. We found increased levels of NGF in the CSF of patients with CNP compared to patients without and to controls. This finding supports the hypothesis that NGF plays a role in MS related CNP.

Distinct TrkA and Ret modulated negative and positive neuropathic behaviors in a mouse model of resiniferatoxin-induced small fiber neuropathy

Neurotrophic factors and their corresponding receptors play key roles in the maintenance of different phenotypic dorsal root ganglion (DRG) neurons, the axons of which degenerate in small fiber neuropathy, leading to various neuropathic manifestations. Mechanisms underlying positive and negative symptoms of small fiber neuropathy have not been systematically explored. This study investigated the molecular basis of these seemingly paradoxical neuropathic behaviors according to the profiles of TrkA and Ret with immunohistochemical and pharmacological interventions in a mouse model of resiniferatoxin (RTX)-induced small fiber neuropathy. Mice with RTX neuropathy exhibited thermal hypoalgesia and mechanical allodynia, reduced skin innervation, and altered DRG expression profiles with decreased TrkA(+) neurons and increased Ret(+) neurons. RTX neuropathy induced the expression of activating transcription factor 3 (ATF3), and ATF3(+) neurons were colocalized with Ret but not with TrkA (P<0.001). As a neuroprotectant, 4-Methylcatechol (4MC) promoted skin reinnervation partially with correlated reversal of the neuropathic behaviors and altered neurochemical expression. Gambogic amide, a selective TrkA agonist, normalized thermal hypoalgesia, and GW441756, a TrkA kinase inhibitor, induced thermal hypoalgesia, which was already reversed by 4MC. Mechanical allodynia was reversed by a Ret kinase inhibitor, AST487, which induced thermal hyperalgesia in naïve mice. The activation of Ret signaling by XIB4035 induced mechanical allodynia and thermal hypoalgesia in RTX neuropathy mice in which the neuropathic behaviors were previously normalized by 4MC. Distinct neurotrophic factor receptors, TrkA and Ret, accounted for negative and positive neuropathic behaviors in RTX-induced small fiber neuropathy, respectively: TrkA for thermal hypoalgesia and Ret for mechanical allodynia and thermal hypoalgesia.

Rats bred for low and high running capacity display alterations in peripheral tissues and nerves relevant to neuropathy and pain

INTRODUCTION

Diet and activity are recognized as modulators of nervous system disease, including pain. Studies of exercise consistently reveal a benefit on pain. This study focused on female rats to understand differences related to metabolic status and peripheral nerve function in females.

METHODS

Here, we investigated parameters of peripheral nerve function relevant to pain in rats selectively bred for high (high-capacity runners; HCR) or low endurance exercise capacity (low-capacity runners; LCR) resulting in divergent intrinsic aerobic capacities and susceptibility for metabolic conditions.

RESULTS

LCR female rats have reduced mechanical sensitivity, higher intraepidermal nerve fiber density and TrkA-positive epidermal axons, increased numbers of Langerhans and mast cells in cutaneous tissues, and a higher fat content despite similar overall body weights compared to female HCR rats. Sensory and motor nerve conduction velocities, thermal sensitivity, and mRNA expression of selected genes relevant to peripheral sensation were not different.

CONCLUSIONS

These results suggest that aerobic capacity and metabolic status influence sensory sensitivity and aspects of inflammation in peripheral tissues that could lead to poor responses to tissue damage and painful stimuli. The LCR and HCR rats should prove useful as models to assess how the metabolic status impacts pain.

The Complement System Component C5a Produces Thermal Hyperalgesia via Macrophage-to-Nociceptor Signaling That Requires NGF and TRPV1

The complement cascade is a principal component of innate immunity. Recent studies have underscored the importance of C5a and other components of the complement system in inflammatory and neuropathic pain, although the underlying mechanisms are largely unknown. In particular, it is unclear how the complement system communicates with nociceptors and which ion channels and receptors are involved. Here we demonstrate that inflammatory thermal and mechanical hyperalgesia induced by complete Freund's adjuvant was accompanied by C5a upregulation and was markedly reduced by C5a receptor (C5aR1) knock-out or treatment with the C5aR1 antagonist PMX53. Direct administration of C5a into the mouse hindpaw produced strong thermal hyperalgesia, an effect that was absent in TRPV1 knock-out mice, and was blocked by the TRPV1 antagonist AMG9810. Immunohistochemistry of mouse plantar skin showed prominent expression of C5aR1 in macrophages. Additionally, C5a evoked strong Ca(2+) mobilization in macrophages. Macrophage depletion in transgenic macrophage Fas-induced apoptosis mice abolished C5a-dependent thermal hyperalgesia. Examination of inflammatory mediators following C5a injection revealed a rapid upregulation of NGF, a mediator known to sensitize TRPV1. Preinjection of an NGF-neutralizing antibody or Trk inhibitor GNF-5837 prevented C5a-induced thermal hyperalgesia. Notably, NGF-induced thermal hyperalgesia was unaffected by macrophage depletion. Collectively, these results suggest that complement fragment C5a induces thermal hyperalgesia by triggering macrophage-dependent signaling that involves mobilization of NGF and NGF-dependent sensitization of TRPV1. Our findings highlight the importance of macrophage-to-neuron signaling in pain processing and identify C5a, NGF, and TRPV1 as key players in this cross-cellular communication.

SIGNIFICANCE STATEMENT

This study provides mechanistic insight into how the complement system, a key component of innate immunity, regulates the development of pain hypersensitivity. We demonstrate a crucial role of the C5a receptor, C5aR1, in the development of inflammatory thermal and mechanical sensitization. By focusing on the mechanisms of C5a-induced thermal hyperalgesia, we show that this process requires recruitment of macrophages and initiation of macrophage-to-nociceptor signaling. At the molecular level, we demonstrate that this signaling depends on NGF and is mediated by the heat-sensitive nociceptive channel TRPV1. This deeper understanding of how immune cells and neurons interact to regulate pain processing is expected to facilitate mechanism-based approaches in the development of new analgesics.

Traumatic brain injury, diabetic neuropathy and altered-psychiatric health: The fateful triangle

Traumatic brain injury is a detrimental medical condition particularly when accompanied by diabetes. There are several comorbidities going along with diabetes including, but not limited to, kidney failure, obesity, coronary artery disease, peripheral vascular disease, hypertension, stroke, neuropathies and amputations. Unlike diabetes type 1, diabetes type 2 is more common in adults who simultaneously suffer from other comorbid conditions making them susceptible to repetitive fall incidents and sustaining head trauma. The resulting brain insult exacerbates current psychiatric disorders such as depression and anxiety, which, in turn, increases the risk of sustaining further brain traumas. The relationship between diabetes, traumatic brain injury and psychiatric health constitutes a triad forming a non-reversible vicious cycle. At the proteomic and psychiatric levels, cellular, molecular and behavioral alterations have been reported with the induction of non-traumatic brain injury in diabetic models such as stroke. However, research into traumatic brain injury has not been systematically investigated. Thus, in cases of diabetic neuropathy complicated with traumatic brain injury, utilizing fine structural and analytical techniques allows the identification of key biological markers that can then be used as innovative diagnostics as well as novel therapeutic targets in an attempt to treat diabetes and its sequelae especially those arising from repetitive mild brain trauma.

The induction of the transcription factor Nrf2 enhances the antinociceptive effects of delta-opioid receptors in diabetic mice

The involvement of heme oxygenase 1 (HO-1) in the modulation of the antinociceptive effects of opioids in type 1 diabetes has been demonstrated but the role played by the transcription factor Nrf2 in the regulation of painful neuropathy and in the effects and expression of δ-opioid receptors (DOR) in type 2 diabetes, has not been studied. In male BKS.Cg-m+/+Leprdb/J (db/db) mice, the anti-allodynic effects produced by a Nrf2 transcription factor activator, sulforaphane (SFN) administered alone and combined with two DOR agonists, [d-Pen(2),d-Pen(5)]-Enkephalin (DPDPE) and (+)-4-[(αR)-α-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N diethylbenzamide (SNC-80), were evaluated. The effects of SFN on glucose levels and body weight as well as on the proteins levels of Nrf2, HO-1, NAD(P)H: quinone oxidoreductase 1 (NQO1), MAPKs (JNK) and DOR in sciatic nerve from db/db mice were also assessed. This study showed that the administration of SFN dose dependently reversed mechanical allodynia, reduced hyperglycemia and body weight gain associated to type 2 diabetes and significantly increased the anti-allodynic effects of DPDPE and SNC-80 in db/db mice. This treatment normalized the down regulation of Nrf2 and NQO1 and enhanced the protein levels of HO-1 in db/db mice. Moreover, the administration of SFN also inhibited the JNK phosphorylation and DOR down-regulation in the sciatic nerve of diabetic mice. Our data indicated that SFN treatment is effective in reversing mechanical allodynia and enhancing DOR antinociceptive effects in db/db mice which effects might be mediated by activating Nrf2 signaling, reducing hyperglycemia, inhibiting JNK phosphorylation and avoiding DOR down-regulation in the sciatic nerve of these animals. These results propose SFN, alone and/or combined with DOR agonists, as interesting approaches for the treatment of painful diabetic neuropathy associated to type 2 diabetes in mice.

Nerve growth factor: a neuroimmune crosstalk mediator for all seasons

Neurotrophic factors comprise a broad family of biomolecules - most of which are peptides or small proteins - that support the growth, survival and differentiation of both developing and mature neurons. The prototypical example and best-characterized neurotrophic factor is nerve growth factor (NGF), which is widely recognized as a target-derived factor responsible for the survival and maintenance of the phenotype of specific subsets of peripheral neurons and basal forebrain cholinergic nuclei during development and maturation. In addition to being active in a wide array of non-nervous system cells, NGF is also synthesized by a range of cell types not considered as classical targets for innervation by NGF-dependent neurons; these include cells of the immune-haematopoietic lineage and populations in the brain involved in neuroendocrine functions. NGF concentrations are elevated in numerous inflammatory and autoimmune states such as multiple sclerosis, chronic arthritis, systemic lupus erythematosus and mastocytosis, in conjunction with increased accumulation of mast cells. Intriguingly, NGF seems to be linked also with diabetic pathology and insulin homeostasis. Mast cells and NGF appear involved in neuroimmune interactions and tissue inflammation. As mast cells are capable of producing and responding to NGF, this suggests that alterations in mast cell behaviour could provoke maladaptive neuroimmune tissue responses, including those of an autoimmune nature. Moreover, NGF exerts a modulatory role on sensory nociceptive nerve physiology in the adult, which appears to correlate with hyperalgesic phenomena occurring in tissue inflammation. NGF can therefore be viewed as a multifactorial modulator of neuro-immune-endocrine functions.

Does Endurance Training Compensate for Neurotrophin Deficiency Following Diabetic Neuropathy?

BACKGROUND

A lack of neurotrophic support is believed to contribute to the development of diabetic neuropathy. On the other hand, neurotrophins have consistently been shown to increase in the central and peripheral nervous system following exercise, but the effects of exercise intervention on brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in diabetic neuropathy are not understood.

OBJECTIVES

This experimental study was designed and carried out at the Tarbiat Modares university (TMU) in Tehran, Iran, to investigate the hypothesis that increased activity as endurance training can help to increase the endogenous expression of neurotrophins in diabetic rats.

METHODS

This was an experimental study with 2 × 2 factorial plans performed at TMU in Iran. Sampling was accidental and 28 adult male Wistar rats in the body mass range of 326.3 ± 8.4 g comprised the sample, with each rat randomly assigned to four groups: diabetic control (DC), diabetic training (DT), healthy control (HC), and healthy training (HT). To induce diabetic neuropathy, after 12 hours of food deprivation, an intraperitoneal injection of streptozotocin (STZ) solution (45 mg/Kg) method was used. Two weeks after STZ injection, the endurance training protocol was performed for 6 weeks; 24 hours after the last training session, the rats were sacrificed. Real-time PCR was used for BDNF and NGF expression.

RESULTS

The data indicate that diabetes decreases BDNF and NGF expression in sensory (92%, P = 0.01; 90%, P = 0.038, respectively) and motor (93%, P = 0.05; 60%, P = 0.029, respectively) roots. However, NGF mRNA levels in the DT group were significantly higher than in the HC group ((7.1-fold), P = 0.01; (2.2-fold), P = 0.001, respectively, for sensory and motor roots), but this was not shown for BDNF. In addition, endurance training can increase NGF expression in healthy rats ((7.4-fold), P = 0.01; (3.8-fold), P = 0.001, respectively, for sensory and motor roots).

CONCLUSIONS

This study shows that BDNF and NGF expression decreases in diabetic neuropathy. However, this decrease can be reversed through endurance training. These results also indicate that endurance training may have a potential role in compensating for neurotrophin deficiency following diabetic neuropathy.

Insights into the pathogenesis and treatment of painful diabetic neuropathy

Painful diabetic distal symmetrical polyneuropathy (painful DPN) is a puzzle with two important missing pieces: Firstly we still do not understand why only some patients with neuropathy experience painful symptoms; Secondly we still do not have a complete understanding of how nociception generated in the peripheral nervous system is processed by the central nervous system (CNS). Available treatments offer only symptom relief and there is currently no effective treatment based on arresting or reversing the progression of disease. Therefore the management of painful DPN remains less than optimal because the complex pathophysiology of nociception and pain perception in health and disease is incompletely understood. Studies of the peripheral nervous system are investigating the molecular processes involved in signal transduction that have the potential to be interrupted or modified to ease pain. Magnetic resonance imaging techniques are helping to elucidate central pain processing pathways and describe the translation of nociception to pain. Combining the knowledge from these two streams of enquiry we will soon be able to predict accurately who will develop painful DPN, how we can halt or reverse the condition, or who will respond to symptomatic treatments. Future developments in the treatment of painful DPN will be underpinned by decoding the peripheral and central mechanisms of pain. Research is focusing on these areas of enquiry in the hope that answers will lead to effective treatments to alleviate pain and reverse pathology for those suffering from painful DPN.

Painful neuropathy: Mechanisms

Painful neuropathy, like the other complications of diabetes, is a growing healthcare concern. Unfortunately, current treatments are of variable efficacy and do not target underlying pathogenic mechanisms, in part because these mechanisms are not well defined. Rat and mouse models of type 1 diabetes are frequently used to study diabetic neuropathy, with rats in particular being consistently reported to show allodynia and hyperalgesia. Models of type 2 diabetes are being used with increasing frequency, but the current literature on the progression of indices of neuropathic pain is variable and relatively few therapeutics have yet been developed in these models. While evidence for spontaneous pain in rodent models is sparse, measures of evoked mechanical, thermal and chemical pain can provide insight into the pathogenesis of the condition. The stocking and glove distribution of pain tantalizingly suggests that the generator site of neuropathic pain is found within the peripheral nervous system. However, emerging evidence demonstrates that amplification in the spinal cord, via spinal disinhibition and neuroinflammation, and also in the brain, via enhanced thalamic activity or decreased cortical inhibition, likely contribute to the pathogenesis of painful diabetic neuropathy. Several potential therapeutic strategies have emerged from preclinical studies, including prophylactic treatments that intervene against underlying mechanisms of disease, treatments that prevent gains of nociceptive function, treatments that suppress enhancements of nociceptive function, and treatments that impede normal nociceptive mechanisms. Ongoing challenges include unraveling the complexity of underlying pathogenic mechanisms, addressing the potential disconnect between the perceived location of pain and the actual pain generator and amplifier sites, and finding ways to identify which mechanisms operate in specific patients to allow rational and individualized choice of targeted therapies.

Resolution of Inflammation by Resolvin D1 Is Essential for Peroxisome Proliferator-activated Receptor-γ-mediated Analgesia during Postincisional Pain Development in Type 2 Diabetes

BACKGROUND

The wound healing process following acute inflammation after surgery is impaired in diabetes. Altered macrophage functions are linked to delayed tissue repair and pain development in diabetes. Although peroxisome proliferator-activated receptor (PPAR)-γ agonists are used to treat diabetes, their postoperative analgesic effects in diabetes have not been evaluated.

METHODS

The PPARγ agonist rosiglitazone (rosi) was injected at the incision site of diabetic (db/db) mice with resolvin (Rv) D1, a lipid mediator involved in resolution of inflammation. Pain-related behavior, neutrophil infiltration, phagocytosis, and macrophage polarity were assessed for 7 days postoperatively.

RESULTS

Rosiglitazone and RvD1 alleviated mechanical hyperalgesia in db/db (db) mice, whereas rosiglitazone alone did not alter mechanical thresholds on days 4 (db rosi + RvD1 vs. db rosi: 0.506 ± 0.106 vs. 0.068 ± 0.12) and 7 (0.529 ± 0.184 vs. 0.153 ± 0.183) after incision (n = 10 per group). In control m/m mice, the rosiglitazone-induced analgesic effects were reversed by knockdown with arachidonate 5-lipoxygenase small interfering RNA, but these were restored by addition of RvD1. In db/db mice treated with rosiglitazone and RvD1, local infiltration of neutrophils was markedly reduced, with an associated decrease in total TdT-mediated dUTP nick-end labeling cells. Acceleration of rosiglitazone-induced phenotype conversion of infiltrated macrophages from M1 to M2 was impaired in db/db mice, but it was effectively restored by RvD1 in db/db wounds.

CONCLUSIONS

In diabetes, exogenous administration of RvD1 is essential for PPARγ-mediated analgesia during development of postincisional pain. Resolution of inflammation accelerated by RvD1 might promote PPARγ-mediated macrophage polarization to the M2 phenotype.

Mechanisms of mesenchymal stem cell correction of the impaired biomechanical properties of diabetic skin: The role of miR-29a

Diabetic skin has impaired wound healing properties following injury. We have further shown that diabetic skin has weakened biomechanical properties at baseline. We hypothesize that the biomechanical properties of diabetic skin decline during the progression of the diabetic phenotype, and that this decline is due to the dysregulation of miR-29a, resulting in decreased collagen content. We further hypothesize that treatment with mesenchymal stem cells (MSCs) may improve diabetic wound healing by correction of the dysregulated miR-29a expression. We analyzed the biomechanical properties, collagen gene expression, collagen protein production, and miR-29a levels in skin harvested from 6 to 18 week old mice during the development of the diabetic phenotype. We also examined the correction of these impairments by both MSC treatment and the inhibition of miR-29a. Diabetic skin demonstrated a progressive impairment of biomechanical properties, decreased collagen content, and increased miR-29a levels during the development of the diabetic phenotype. MSC treatment decreased miR-29a levels, increased collagen content, and corrected the impaired biomechanical properties of diabetic skin. Additionally, direct inhibition of miR-29a also increased collagen content in diabetic skin. This decline in the biomechanical properties of diabetic skin during the progression of diabetes may increase the susceptibility of diabetic skin to injury and miR-29a appears to play a key role in this process.

Axon Transport and Neuropathy: Relevant Perspectives on the Etiopathogenesis of Familial Dysautonomia

Peripheral neuropathies are highly prevalent and are most often associated with chronic disease, side effects from chemotherapy, or toxic-metabolic abnormalities. Neuropathies are less commonly caused by genetic mutations, but studies of the normal function of mutated proteins have identified particular vulnerabilities that often implicate mitochondrial dynamics and axon transport mechanisms. Hereditary sensory and autonomic neuropathies are a group of phenotypically related diseases caused by monogenic mutations that primarily affect sympathetic and sensory neurons. Here, I review evidence to indicate that many genetic neuropathies are caused by abnormalities in axon transport. Moreover, in hereditary sensory and autonomic neuropathies. There may be specific convergence on gene mutations that disrupt nerve growth factor signaling, upon which sympathetic and sensory neurons critically depend.

Chronic stress and peripheral pain: Evidence for distinct, region-specific changes in visceral and somatosensory pain regulatory pathways

Chronic stress alters the hypothalamic-pituitary-adrenal (HPA) axis and enhances visceral and somatosensory pain perception. It is unresolved whether chronic stress has distinct effects on visceral and somatosensory pain regulatory pathways. Previous studies reported that stress-induced visceral hyperalgesia is associated with reciprocal alterations of endovanilloid and endocannabinoid pain pathways in DRG neurons innervating the pelvic viscera. In this study, we compared somatosensory and visceral hyperalgesia with respect to differential responses of peripheral pain regulatory pathways in a rat model of chronic, intermittent stress. We found that chronic stress induced reciprocal changes in the endocannabinoid 2-AG (increased) and endocannabinoid degradation enzymes COX-2 and FAAH (decreased), associated with down-regulation of CB1 and up-regulation of TRPV1 receptors in L6-S2 DRG but not L4-L5 DRG neurons. In contrast, sodium channels Nav1.7 and Nav1.8 were up-regulated in L4-L5 but not L6-S2 DRGs in stressed rats, which was reproduced in control DRGs treated with corticosterone in vitro. The reciprocal changes of CB1, TRPV1 and sodium channels were cell-specific and observed in the sub-population of nociceptive neurons. Behavioral assessment showed that visceral hyperalgesia persisted, whereas somatosensory hyperalgesia and enhanced expression of Nav1.7 and Nav1.8 sodium channels in L4-L5 DRGs normalized 3 days after completion of the stress phase. These data indicate that chronic stress induces visceral and somatosensory hyperalgesia that involves differential changes in endovanilloid and endocannabinoid pathways, and sodium channels in DRGs innervating the pelvic viscera and lower extremities. These results suggest that chronic stress-induced visceral and lower extremity somatosensory hyperalgesia can be treated selectively at different levels of the spinal cord.

The Metabolic Syndrome and Microvascular Complications in a Murine Model of Type 2 Diabetes

To define the components of the metabolic syndrome that contribute to diabetic polyneuropathy (DPN) in type 2 diabetes mellitus (T2DM), we treated the BKS db/db mouse, an established murine model of T2DM and the metabolic syndrome, with the thiazolidinedione class drug pioglitazone. Pioglitazone treatment of BKS db/db mice produced a significant weight gain, restored glycemic control, and normalized measures of serum oxidative stress and triglycerides but had no effect on LDLs or total cholesterol. Moreover, although pioglitazone treatment normalized renal function, it had no effect on measures of large myelinated nerve fibers, specifically sural or sciatic nerve conduction velocities, but significantly improved measures of small unmyelinated nerve fiber architecture and function. Analyses of gene expression arrays of large myelinated sciatic nerves from pioglitazone-treated animals revealed an unanticipated increase in genes related to adipogenesis, adipokine signaling, and lipoprotein signaling, which likely contributed to the blunted therapeutic response. Similar analyses of dorsal root ganglion neurons revealed a salutary effect of pioglitazone on pathways related to defense and cytokine production. These data suggest differential susceptibility of small and large nerve fibers to specific metabolic impairments associated with T2DM and provide the basis for discussion of new treatment paradigms for individuals with T2DM and DPN.

Neurotrophins and Neuropathic Pain: Role in Pathobiology

Neurotrophins (NTs) belong to a family of trophic factors that regulate the survival, growth and programmed cell death of neurons. In mammals, there are four structurally and functionally related NT proteins, viz. nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 and neurotrophin 4. Most research on NTs to date has focussed on the effects of NGF and BDNF signalling via their respective cognate high affinity neurotrophic tyrosine kinase viz TrkA and TrkB receptors. Apart from the key physiologic roles of NGF and BDNF in peripheral and central nervous system function, NGF and BDNF signalling via TrkA and TrkB receptors respectively have been implicated in mechanisms underpinning neuropathic pain. Additionally, NGF and BDNF signalling via the low-affinity pan neurotrophin receptor at 75 kDa (p75NTR) may also contribute to the pathobiology of neuropathic pain. In this review, we critically assess the role of neurotrophins signalling via their cognate high affinity receptors as well as the low affinity p75NTR in the pathophysiology of peripheral neuropathic and central neuropathic pain. We also identify knowledge gaps to guide future research aimed at generating novel insight on how to optimally modulate NT signalling for discovery of novel therapeutics to improve neuropathic pain relief.

Mechanisms of distal axonal degeneration in peripheral neuropathies

Peripheral neuropathy is a common complication of a variety of diseases and treatments, including diabetes, cancer chemotherapy, and infectious causes (HIV, hepatitis C, and Campylobacter jejuni). Despite the fundamental difference between these insults, peripheral neuropathy develops as a combination of just six primary mechanisms: altered metabolism, covalent modification, altered organelle function and reactive oxygen species formation, altered intracellular and inflammatory signaling, slowed axonal transport, and altered ion channel dynamics and expression. All of these pathways converge to lead to axon dysfunction and symptoms of neuropathy. The detailed mechanisms of axon degeneration itself have begun to be elucidated with studies of animal models with altered degeneration kinetics, including the slowed Wallerian degeneration (Wld(S)) and Sarm knockout animal models. These studies have shown axonal degeneration to occur through a programmed pathway of injury signaling and cytoskeletal degradation. Insights into the common disease insults that converge on the axonal degeneration pathway promise to facilitate the development of therapeutics that may be effective against other mechanisms of neurodegeneration.

Upregulation of T-type Ca2+ channels in long-term diabetes determines increased excitability of a specific type of capsaicin-insensitive DRG neurons

Background

Previous studies have shown that increased excitability of capsaicin-sensitive DRG neurons and thermal hyperalgesia in rats with short-term (2–4 weeks) streptozotocin-induced diabetes is mediated by upregulation of T-type Ca²⁺ current. In longer–term diabetes (after the 8th week) thermal hyperalgesia is changed to hypoalgesia that is accompanied by downregulation of T-type current in capsaicin-sensitive small-sized nociceptors. At the same time pain symptoms of diabetic neuropathy other than thermal persist in STZ-diabetic animals and patients during progression of diabetes into later stages suggesting that other types of DRG neurons may be sensitized and contribute to pain. In this study, we examined functional expression of T-type Ca²⁺ channels in capsaicin-insensitive DRG neurons and excitability of these neurons in longer-term diabetic rats and in thermally hypoalgesic diabetic rats.

Results

Here we have demonstrated that in STZ-diabetes T-type current was upregulated in capsaicin-insensitive low-pH-sensitive small-sized nociceptive DRG neurons of longer-term diabetic rats and thermally hypoalgesic diabetic rats. This upregulation was not accompanied by significant changes in biophysical properties of T-type channels suggesting that a density of functionally active channels was increased. Sensitivity of T-type current to amiloride (1 mM) and low concentration of Ni²⁺ (50 μM) implicates prevalence of Ca_v3.2 subtype of T-type channels in the capsaicin-insensitive low-pH-sensitive neurons of both naïve and diabetic rats. The upregulation of T-type channels resulted in the increased neuronal excitability of these nociceptive neurons revealed by a lower threshold for action potential initiation, prominent afterdepolarizing potentials and burst firing. Sodium current was not significantly changed in these neurons during long-term diabetes and could not contribute to the diabetes-induced increase of neuronal excitability.

Conclusions

Capsaicin-insensitive low-pH-sensitive type of DRG neurons shows diabetes-induced upregulation of Ca_v3.2 subtype of T-type channels. This upregulation results in the increased excitability of these neurons and may contribute to nonthermal nociception at a later-stage diabetes.

Mouse models of diabetic neuropathy

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes and is associated with significant morbidity and mortality. DPN is characterized by progressive, distal-to-proximal degeneration of peripheral nerves that leads to pain, weakness, and eventual loss of sensation. The mechanisms underlying DPN pathogenesis are uncertain, and other than tight glycemic control in type 1 patients, there is no effective treatment. Mouse models of type 1 (T1DM) and type 2 diabetes (T2DM) are critical to improving our understanding of DPN pathophysiology and developing novel treatment strategies. In this review, we discuss the most widely used T1DM and T2DM mouse models for DPN research, with emphasis on the main neurologic phenotype of each model. We also discuss important considerations for selecting appropriate models for T1DM and T2DM DPN studies and describe the promise of novel emerging diabetic mouse models for DPN research. The development, characterization, and comprehensive neurologic phenotyping of clinically relevant mouse models for T1DM and T2DM will provide valuable resources for future studies examining DPN pathogenesis and novel therapeutic strategies.

Neural mobilization promotes nerve regeneration by nerve growth factor and myelin protein zero increased after sciatic nerve injury

Neurotrophins are crucial in relation to axonal regrowth and remyelination following injury; and neural mobilization (NM) is a noninvasive therapy that clinically is effective in neuropathic pain treatment, but its mechanisms remains unclear. We examined the effects of NM on the regeneration of sciatic nerve after chronic constriction injury (CCI) in rats. The CCI was performed on adult male rats, submitted to 10 sessions of NM, starting 14 days after CCI. Then, the nerves were analyzed using transmission electron microscopy and western blot for neural growth factor (NGF) and myelin protein zero (MPZ). We observed an increase of NGF and MPZ after CCI and NM. Electron microscopy revealed that CCI-NM samples had high numbers of axons possessing myelin sheaths of normal thickness and less inter-axonal fibrosis than the CCI. These data suggest that NM is effective in facilitating nerve regeneration and NGF and MPZ are involved in this effect.

Biology of diabetic neuropathy

More than half of all patients with diabetes develop neuropathic disorders affecting the distal sensory and/or motor nerves, or autonomic or cranial nerve functions. Glycemic control can decrease the incidence of neuropathy but is not adequate alone to prevent or treat the disease. This chapter introduces diabetic neuropathy with a morphological description of the disease then describes our current understanding of metabolic and molecular mechanisms that contribute to neurovascular dysfunctions. Key mechanisms include glucose and lipid imbalances and insulin resistance that are interconnected via oxidative stress, inflammation, and altered gene expression. These complex interactions should be considered for the development of new treatment strategies against the onset or progression of neuropathy. Advances in understanding the combined metabolic stressors and the novel study of epigenetics suggest new therapeutic targets to combat this morbid and intractable disease affecting millions of patients with type 1 or type 2 diabetes.

Animal models of diabetes-induced neuropathic pain

Neuropathy will afflict over half of the approximately 350 million people worldwide who currently suffer from diabetes and around one-third of diabetic patients with neuropathy will suffer from painful symptoms that may be spontaneous or stimulus evoked. Diabetes can be induced in rats or mice by genetic, dietary, or chemical means, and there are a variety of well-characterized models of diabetic neuropathy that replicate either type 1 or type 2 diabetes. Diabetic rodents display aspects of sensorimotor dysfunction such as stimulus-evoked allodynia and hyperalgesia that are widely used to model painful neuropathy. This allows investigation of pathogenic mechanisms and development of potential therapeutic interventions that may alleviate established pain or prevent onset of pain.